1. Field of the Invention
The present invention relates to a projection optical system, and more particularly to a projection optical system that incorporates reflective and refractive optical elements in an optical construction suitable for rear projection.
2. Description of Related Art
In a projection optical system for performing wide-angle enlargement projection from a primary image surface on the reduction side to a secondary image surface on the enlargement side, disposing in the optical path a negative mirror closer to the secondary image surface is effective in reducing chromatic aberration and distortion. Projection optical systems including a negative mirror for reducing chromatic and other aberrations are proposed, for example, in Patent Publications 1 to 3 listed below. Patent Publication 1 proposes a projection optical system in which there are arranged, from the primary image surface side, a refractive lens group, a flat mirror, a negative mirror, and a flat mirror. This construction helps realize a wide-angle, high-performance projection optical system and a slim projection apparatus. Moreover, here, for enhanced optical performance, reflective surfaces are formed as non-rotation-symmetric free-form, anamorphic aspherical, rotation-symmetric aspherical, or other surfaces.
According to Patent Publication 2, there are arranged, from the primary image surface side, a refractive lens group (whose optical axis is perpendicular to the normal vector to the secondary image surface), a flat mirror, a negative mirror, and a flat mirror. This construction helps realize a wide-angle, high-performance projection optical system and a slim projection apparatus. Moreover, the refractive lens group is so designed that, in the part thereof located on the enlargement side of the aperture stop provided therein, the radius of the circle that encircles all the image light rays as observed at each surface provided in that part of the refractive lens group first increases and the decreases. This helps lessen the large positive Petzval sum produced by the negative mirror and thereby achieve still higher performance. Moreover, for enhanced optical performance, the reflective surface of the negative mirror is formed as a non-rotation-symmetric free-form surface.
According to Patent Publication 3, there are arranged, from the primary image surface side, a refractive lens group (whose optical axis is perpendicular to the normal vector to the secondary image surface), a fourth mirror having a flat reflective surface, a third mirror having a negative or positive optical power, a second mirror having a negative or positive optical power or a flat reflective surface, and a first mirror having a flat reflective surface. This helps realize a slim projection apparatus. Using two curved-surface mirrors helps reduce the Petzval sum, permitting the eccentric aberration produced by each mirror to be compensated for by the other. Moreover, the eccentric aberration produced by the third mirror is corrected for by an eccentricity correction lens element provided in the refractive lens group, and, for enhanced optical performance, the reflective surfaces of the negative and positive mirrors are formed as aspherical or free-form surfaces.                Patent Publication 1: Japanese Patent Application Laid-Open No. 2001-264627        Patent Publication 2: U.S. Pat. No. 6,690,517        Patent Publication 3: Japanese Patent Application Laid-Open No. 2002-341452        
The projection optical systems proposed in Patent Publications 1 and 2 achieve a wide angle by the use of reflective and refractive optical elements. Here, if the absolute value of the optical power of the negative mirror can be increased, it is possible to achieve a wider angle. However, increasing the absolute value of the optical power of the negative mirror causes the Petzval sum to become lopsided in the positive direction. Thus, to secure satisfactory image surface flatness, the Petzval sum needs to be shifted back in the negative direction, and this requires that, somewhere within the entire optical system, the absolute value of the optical power of a refractive lens element having a negative optical power be increased. This, however, cannot be done without lengthening the focal length of the refractive lens group, and thus brings an effect contrary to a wider angle. Moreover, in a telecentric system, it is essential to dispose in the optical path a lens element having a strong positive optical power (causing the Petzval sum to be shifted in the positive direction) on the primary image surface side of the refractive lens group, and this makes it difficult to achieve a wider angle.
Another way to achieve a wider angle is to use larger reflective optical elements and adopt a construction that permits distortion to be reduced more easily. In this construction, however, the light beam needs to be made to diverge by the refractive lens group located on the primary image surface side of the reflective optical elements. This requires that the interval between the refractive lens group and the reflective optical elements be increased, or that the effective diameter of the refractive lens group be increased, or that the number of lens elements provided in the refractive lens group be increased. Doing any of these eventually makes the projection apparatus larger and increases costs.
Moreover, when use as a rear projection apparatus is considered, to make the apparatus as a whole slim, the optical path needs to be bent with a flat mirror or the like disposed between the refractive lens group and the reflective optical elements (see paragraph 0019 of Patent Publication 1). However, as described above, the use of larger reflective optical elements and of a wider-angle refractive lens group makes it extremely difficult to lay out the optical path inside the rear projection apparatus, and thus makes it impossible to achieve a wider angle. Moreover, to dispose a flat mirror without interference with other optical elements or the optical path, it is necessary to secure a large space between the refractive lens group and the negative mirror. This increases the optical path length through the projection optical system, and is thus unsuitable for aiming at a wider angle or making the projection apparatus slim.
The projection optical systems proposed in Patent Publications 1 and 2 reduce aberrations by giving the optical surface of the negative mirror a non-rotation-symmetric shape. Such a shape, however, cannot be produced by turning, and its production and evaluation require sophisticated technologies as compared with a rotation-symmetric shape. This may lead to higher costs.
With the projection optical system proposed in Patent Publication 3, a wider angle and further slimness are possible thanks to the second and third mirrors having optical powers and the eccentricity correction lens element. The Petzval sum, if shifted in the positive direction by a negative mirror, can be shifted back in the negative direction with a positive mirror, but those are not achieved because of the other part of the construction and the inevitable restrictions. The first reason relates to, as in the projection optical systems disclosed in Patent Publications 1 and 2, the presence of a flat mirror as the fourth mirror. An attempt to bend the optical path with the fourth mirror without interference ends in securing a large space between the refractive optical system and the third mirror. This makes the optical path length greater and requires larger optical elements, and is thus unsuitable for aiming at a wider angle and further compactness. The second reason relates to the inclination of the first mirror with respect to the screen. This relates to the small launch angle θ (see FIG. 2 of Patent Publication 3). That is, the difference in optical path length between, of the principal rays traveling from the third mirror to the screen, those reaching the top end of the screen and those reaching the bottom end thereof (the ±Y-direction ends in FIG. 2 of Patent Publication 3) is small. Thus, this construction hardly differs from a coaxial optical system whose optical path is bent a plurality of times simply with flat reflective surfaces, and is thus unsuitable for aiming at a wider angle and further slimness. Moreover, the inclination of the first mirror with respect to the screen shortens the optical path length at the top end of the screen, making the construction unsuitable for aiming at a wider angle by increasing the degree of eccentricity or of shifting.
Moreover, the projection optical system proposed in Patent Publication 3 adopts an optical construction in which the display device and the second and third mirrors are three-dimensionally, rotationally eccentric. Thus, all the rays reaching the screen have no plane of symmetry at all. This makes it difficult to correct for aberrations, and is thus unsuitable for aiming at higher performance. Moreover, as increasingly wider angles are aimed at, the magnification of the projection optical system increases, and the second and third mirrors become larger as measured on the YZ plane (see FIG. 2 of the Patent Publication 3), making them difficult to produce and increasing costs. Thus, this construction is unsuitable for aiming at a wider angle, further slimness, and lower cost.